1. Field of the Invention
The present invention relates, in general, to metal making apparatus and, specifically, to molten metal receptacles and, more specifically, to slag control shapes used in molten metal vessels.
2. Description of the Art
In metal making processes, such as steel making, molten metal is transferred from a furnace or converter by a ladle to a tundish or directly to a casting machine. In all metal making processes, and, in particular, in steel making processes, a layer of slag containing metal impurities forms above the top surface of the molten metal within the ladle and the tundish. When the molten metal is discharged from the ladle or tundish, it is necessary to maintain a separation between the slag and the molten metal so that high quality steel without significant amounts of slag can be produced.
The slag forms a layer of impurities several inches thick on top of the layer of molten metal in the ladle and in the tundish. In addition, the flow of molten metal through the discharge nozzle in the ladle or tundish creates a vortex which introduces a conically-shaped rotation to the molten metal immediately above the discharge nozzle. When a sufficient quantity of molten metal is maintained within the ladle or tundish, the vortex forms completely within the molten metal layer and does not reach to the slag layer atop the molten metal layer. However, when the level of molten metal within the ladle or tundish drops below the predetermined critical depth, the vortex reaches into the slag layer and draws slag through the center of the vortex to the discharge nozzle along with molten metal. This causes the introduction of slag into the molten metal as it is discharged from the ladle or the tundish and results in steel having less than desirable quantities as well as creating a potentially hazardous situation.
In order to prevent the introduction of slag into the molten metal, various slag control shapes, such as balls, frusto-conical bodies, etc., as shown in U.S. Pat. Nos. 4,725,045 and 4,968,007, are introduced into the transfer ladle or tundish. Such slag control shapes or bodies have a predetermined specific gravity less than the specific gravity of the molten metal and greater than the specific gravity of the slag layer so that the slag control shape or body is buoyantly supported at the interface between the slag layer and the molten metal layer. Such slag control bodies or shapes are also designed to locate and center themselves automatically in the vortex formed above the discharge nozzle from the molten metal vessel or receptacle. The lower portion of such slag control bodies is disposed in the molten metal layer and will enter and seat within the upper portion of the discharge nozzle of the molten metal receptacle when the molten metal layer drops below a predetermined depth so as to block the discharge nozzle and prevent the discharge of slag from the receptacle.
While such slag control bodies or shapes have found widespread use and effectively block the undesirable discharge of slag from a molten metal vessel, such as a transfer ladle or tundish, the introduction of such slag control bodies into the molten metal receptacle has proved to be a problem.
Typically, such slag control bodies are introduced into the transfer ladle or tundish at a predetermined time during the discharge of molten metal from the ladle or tundish. The time of insertion of the slag control body is based on an operator's experience, typically on the total time of molten metal discharge, or on a potentially inaccurate vessel or ladle weight reading from a load cell in a ladle weight system. As ladles positioned in caster turret arms are typically 20 feet or more in height, overhead cranes have been used to drop the slag control body into the ladle at the point in time indicated by an operator. However, such cranes are assigned numerous other tasks which make it difficult to insure that a crane is available at the precise time that the operator determines it necessary to insert the slag control body into the molten metal vessel.
A small number of metal making or casting machine installations have a stairway located adjacent the discharge position of a ladle which enables a worker, such as a ladleman, to climb to the top of the ladle and insert the slag control body into the ladle at the required time. However, the height of the ladle, the approximate 25 pounds or more weight of the slag control body, and the high temperatures involved in the molten metal process make such a task difficult, undesirable and dangerous. Further, the ladleman typically has other duties in monitoring the metal making process which must be neglected for the time it takes to climb the stairs and insert the slag control body. Dedicating one person solely to the task of inserting the slag control body into the molten metal vessel at the required time adds costs to the metal making process as such an individual is only required is to perform his single task at widely spaced, intermittent intervals.
Further, when such slag control shapes are dropped into a molten metal vessel, they typically fall from 10 to 15 feet before hitting the slag layer. Due to the buoyancy characteristics of a slag control shape and its momentum during dropping into the vessel, the slag control shape will initially pass through the slag layer and into the molten metal and then bob up out of the molten metal and slag until it settles at the molten metal/slag interface. However, this bobbing force and the inherent buoyancy characteristics of a slag control shape frequently cause the slag control shape to settle at a position away from a desired position directed above the discharge outlet of the molten metal vessel. Indeed, it is infrequent for the slag control shape to settle directly over the discharge outlet since the discharge outlet is typically 21/2 to 41/2 inches in diameter as compared to the 10 to 20 foot diameter of a typical ladle. Thus, when a vortex begins to form above the discharge outlet when the molten metal reaches a low level within the ladle or vessel, the slag control shape may not be able to reach the vortex in time to serve its function of blocking the outlet to prevent the discharge of slag through the outlet. Furthermore, even if the slag control shape initially settles directly over the discharge outlet, it frequently drifts away since a vortex may not have formed above the outlet and never returns to the desired centered position thereby defeating its intended purpose.
In order to address these problems, the Applicants' previously devised a slag control shape release apparatus which is disclosed in U.S. Pat. No. 5,249,780 issued on Oct. 5, 1993. In this apparatus, the hanger of a slag control shape extends through a bore in a molten metal vessel cover and, also, through a bore in a lid pivotally mounted on the cover. A pin is mounted on the lid and biased to a position extending through the hanger to support the slag control shape on the cover. An actuating cable is connected to the pin and extends from the cover to an easily accessible position for remote actuation of the release mechanism to release the pin from the slag control shape and to allow the descent of the slag control shape into the molten metal vessel.
A similar release apparatus has also been devised by the Applicants and is disclosed in U.S. Pat. No. 5,303,902 issued on Apr. 19, 1994. In this apparatus, a mounting means including a reciprocally movable pin is mounted on a frame pivotally mounted on the cover of a molten metal vessel and is actuated by a cable extending from the pin to a free end remote from the cover. A cable wound around a reel mounted on the frame is attached to the slag control shape to control the descent of the slag control shape into the molten metal vessel after release from the pin.
Yet another release apparatus has been devised by the Applicants' and is disclosed in co-pending U.S. patent application Ser. No. 08/196,309, filed on Feb. 15, 1994. In this latter release apparatus, a lid is removably implacable in a bore in the cover. The hanger or rod of a slag control shape extends through the lid and is releasably received in mounting means on the lid. The actuating means is similar to that in the other release devices devised by the Applicants' in that it is in the form of a cable having an end operable from a location remote from the cover on the molten metal receptacle. In this latter design, means are provided for releasably connecting the actuating means to the mounting means after the mounting means and the lid have been mounted on the cover. This enables the heavy mounting and activating means to be mounted on the lid rather than on the cover which is typically formed of lightweight refractory or ceramic fiber materials.
While all of these apparatus have proved effective at accurately dropping a slag control shape into a molten metal vessel and permitting the timely release of the slag control shape from an easily accessible location remote from the cover on top of a molten metal vessel, such release apparatus still require the manual intervention of the ladleman or operator at the proper time to pull the cable to release the slag control shape from the cover. Such manual intervention still requires the operator's experience to determine the proper time to release the slag control shape, which is usually based on the total time of metal flow from the receptacle or on a scale weight reading to provide an indication of the amount of molten metal remaining in the receptacle.
Thus, it would be desirable to provide a control apparatus for automatically inserting a slag control shape into a molten metal vessel at a predetermined time during a molten metal forming operation. It would also be desirable to provide a control apparatus for automatically inserting a slag control shape which can be mounted on the lightweight cover currently used with a molten metal vessel.